The linear Fresnel lens is a renewable source of energy which is designed according to the Fermat’s principle. According to this principle it is said that the lens is slightly modified with respect to used technology for mass production from glass. A Fresnel lens was originally developed by French physicist Augustin – Jean Fresnel for light houses.
Fresnel lens we first used in 1880s as the lens that focuses beam in light house. Plastic Fresnel lens are used as magnifiers. The quality of image through Fresnel lens is not as good as magnifiers yet in many applications, perfect image quality is not necessary.
In 1882, the French physicist, Augustin invented this lens. It is a first order lens made in France and shipped to United States in mid of nineteenth century. This lens has 1,176 prisms and 24 bullseyes. Magnifying glass is placed at the center so that the light beam is even more powerful. The sections of lens is held together by brass frames which weigh five to six tons each. Also the light through this could be seen from more than 20 miles.
The fundamental idea behind Fresnel lens is very simple. The Fresnel lens lessens the material required as compared to conventional lens. It divides the lens into the set of concentric annular sections say in the form of rings. Each ring is slightly thinner than the next. It focuses light at the center. Now modify each ring so that it’s flat on one side and make it the same thickness as others. Ensure that each ring’s angled face will be different. Now is you stack all the rings back together there comes the result as Fresnel lens. The lens can be made large as per requirement. Large Fresnel lens are usually used as solar concentrates.
Theory of Fresnel lens:
The principle behind the concept of Fresnel lens is that direction of propagation of light does not change within a medium. The material in the center of the lens serves only to increase the amount of weight and absorption with in the system. To take the advantage of above physical property , experiments began which as a result is now known as Fresnel lens.
Types of Fresnel lens:
Basically there are two types of Fresnel lens:
- Imaging Fresnel lens
- Non Imaging Fresnel lens
Under Imaging Fresnel lens there are sub categories:
- Spherical lens: It focuses light on single point. This lens produces sharp images although it is not very clear.
- Cylindrical lens: It uses multiple cylindrical arcs and focus light on single line. Due to diffraction at the edges, the images are not clear.
Under non imaging Fresnel lens also there are two sub divisions:
- Spot lens: It uses flat segments and focus light on small spot. It does not produce sharp images but are applied in solar applications.
- Linear lens: segments are arranged linearly focusing light on a narrow band.
- It consists of series of concentric grooves.
- They have thin light weight construction.
- Excellent light gathering ability for example: condenser.
- They can also be used as magnifiers or projection lenses.